Apparatus presently exists for simultaneously exposing and irradiating the opposite surfaces of a workpiece. For example, see U.S. Pat. No. 3,655,173 issued Apr. 11, 1972.
One specific application of the method and apparatus of the present invention relates to the production of printed wiring boards (hereinafter "PWBs" or, simply, "boards") and more particularly to radiation treatment of solder mask films applied to the surface (or surfaces) of PWBs.
"Solder mask" is a term utilized to identify a coating applied to the surface of a PWB which has the unique property of preventing solder from making contact with predefined surface areas on the PWB during soldering or solder coating processes, which predefined surfaces are coated with a "solder mask".
Solder mask may be applied to PWBs by a variety of methods including, but not limited to, screen printing, wet photo-imaging, and dry film photo-imaging. The apparatus and method of the present invention may be used to great advantage in practicing the dry film solder masking technique (hereinafter referred to as "DFSM"). However, the present invention may also find advantageous application in other similar processes.
Performance of the DFSM technique includes a series of process steps for applying the solder mask film and establishing the ultimate desired image or pattern upon one or both surfaces of a PWB. The specific steps of the DFSM technique are well known to those having ordinary skill in the solder masking field and will not be repeated herein for purposes of brevity. For the purpose of understanding the present invention and the advantages derived from the utilization of the present invention in the performance of the DFSM technique it is sufficient to understand that the final step in the process is irradiation of the film with ultraviolet (UV) radiant energy which promotes cross-linking of the polymers utilized in the masking film and for establishing the desired physical characteristics of the film.
In performing the irradiation process it is often required that opposing surfaces of the workpiece be irradiated, especially when both surfaces are coated with the solder mask material. It is further desirable to perform the irradiation process with minimum temperature rise in the workpiece to mitigate temperature induced shock and/or thermal expansion of components comprising the PWB. It is further desirable to simultaneously irradiate both surfaces of the PWB to achieve maximum efficiency of space and/or machinery utilization and to minimize manipulation and/or handling of the workpiece.
Irradiation of surfaces is a well known art which takes a variety of forms and functions. In the employment of the DFSM technique, it is desirable to have the workpiece travel continuously from workpiece loading to UV processing to removal and collection while oriented horizontally upon a conveying mechanism.
A conveying mechanism suitable for use in practicing the present invention is described in U.S. Pat. No. 3,655,173 issued Apr. 11, 1972 in the name of the present inventor and assigned to the assignee of the present invention. The conveyor described therein is of an open mesh type enabling radiation to substantially pass therethrough and impinge upon a workpiece substantially without obstruction due to the strands of the open mesh structure. Selected strands of the conveyor of the aforementioned U.S. Patent are provided with supporting projections (hereinafter referred to as "contacts") which maintain the workpiece a displaced distance above the general plane of the open mesh conveyor and which cooperate with the irradiation devices to minimize the shadowing effect of the mesh upon the surface of the workpiece.
Although the total surface contact between the workpiece and the mesh conveyor is limited to the aforesaid contacts, the workpiece nevertheless rests upon and engages a plurality of contacts which remain in contact with the PWB during the duration of conveyance, subjecting the support points and the immediately surrounding areas to experience some difference in radiation compared with those areas of the PWB displaced from the conveyor belt contacts resulting in non-uniform radiation and heating which can cause significant differences in physical properties between the points of support and the remaining surface area.